Such a device is known from the article "30-AX Self-aligning 110.degree. in line colour t.v. display" in Electronic Components and Applications Vol. 1, No. 2, February 1979. The magnetized means for maintaining a static magnetic multi-pole field near the output of the electron gun system consist in the device described in this article of a thin magnetized ring of a magnetic half-hard material, which ring is provided in the end of the gun. Dependent on the occurring convergence, colour purity and frame defects, the ring in the finished tube is magnetized to form a multipole (a combination of 2, 4 and 6 pole fields), so that the errors are corrected, as is described elaborately in U.S. Pat. No. 4,220,897 which may be considered to be incorporated herein by reference. Although in principle this described device forms a completely corrected system, it nevertheless appears in practice that small static and dynamic convergence errors occur during the operation of the devices. These errors may be the result, for example, of errors during the magnetization of the ring or of the use of other operating voltages during operation of the display tube. Small errors in the horizontal static convergence are intensified upon horizontal deflection and may adversely cooperate with the dynamic convergence errors of the coil.